A polymer film of typically cellulose ester, polyester, polycarbonate, cyclo-olefin polymer, vinyl polymer or polyimide is used in silver halide photographic materials, retardation films, polarizers and image display devices. Films that are more excellent in point of the surface smoothness and the uniformity can be produced from these polymers, and the polymers are therefore widely employed for optical films.
Of those, cellulose ester films having suitable moisture permeability can be directly stuck to most popular polarizing films formed of polyvinyl alcohol (PVA)/iodine in on-line operation. Accordingly, cellulose acylate, especially cellulose acetate is widely employed as a protective film for polarizers.
On the other hand, when transparent polymer films are applied to optical use, for example, in retardation films, supports for retardation films, protective films for polarizers and liquid crystal display devices, the control of their optical anisotropy is an extremely important element in determining the performance (e.g., visibility) of display devices. With the recent demand for broadening the viewing angle of liquid crystal display devices, improvement of retardation compensation in the devices is desired, for which it is desired to suitably control the in-plane retardation Re (this may be simply referred to as Re) and the thickness-direction retardation Rth (this may be simply referred to as Rth) of the retardation film to be disposed between a polarizing film and a liquid crystal cell. In particular, since transparent polymer films having a negative Rth and satisfying Rth/Re≧−0.39 are not easy to produce, and it is desired to produce them in a simplified manner.
For producing transparent polymer films having a negative Rth, disclosed is a method of forming a vertically-aligned liquid crystal layer on an isotropic transparent polymer film serving as a support (e.g., see JP-A-6-331826), but the method is problematic in that the production process according to it is complicated and that the producibility of the alignment.
For producing a transparent polymer film having a negative Rth, also disclosed is a method of utilizing a cellulose ester film having a high substitution degree of acetyl group (e.g., see JP-A-2005-120352). This method can give a film having suitable moisture permeability, but it is problematic in that a large amount of energy is necessary for dissolving the polymer and Re does not express sufficiently.
Further, for producing a transparent polymer film having Rth/Re≧−0.5, disclosed is a continuous production method in which a transparent polymer film is adhered with a heat-shrinkable film and subjected to heating and stretching treatment (e.g., see JP-A-5-157911, JE-A-2000-231016). But the method is problematic in that a large amount of heat-shrinkable films are consumed in the practice of the method, and that, in addition, the variation occurs in the quality of the obtained film. The problem is especially distinct for a polymer having a high modulus of elasticity such as cellulose ester.